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How does a Capacitor Work?

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Think of water flowing through a pipe. If we imagine a capacitor as being a storage tank with an inlet and an outlet pipe, it is possible to show approximately how an electronic capacitor works.

First, let's consider the case of a "coupling capacitor" where the capacitor is used to connect a signal from one part of a circuit to another but without allowing any direct current to flow.

	If the current flow is alternating between zero and a maximum, our "storage tank" capacitor will allow the current waves to pass through.
	However, if there is a steady current, only the initial short burst will flow until the "floating ball valve" closes and stops further flow.
	A coupling capacitor is represented like this

So a coupling capacitor allows "alternating current" to pass through because the ball valve doesn't get a chance to close as the waves go up and down. However, a steady current quickly fills the tank so that all flow stops.

A capacitor will pass alternating current but (apart from an initial surge) it will not pass d.c.

 

	Where a capacitor is used to decouple a circuit, the effect is to "smooth out ripples". Any ripples, waves or pulses of current are passed to ground while d.c. flows smoothly.
	A decoupling capacitor is represented like this

What does a capacitor look like?

	"Plug-in" electrolytic used in power supplies where a high ripple-current must be tolerated. The large size indicates either high voltage rating or high capacitance value. This example is 47uF rated at 400 volts dc
	Reservoir electrolytic capacitor used in power supplies where a large capacitance value is required. Poor tolerance, poor temperature stability and polarised. This example is 2200uF rated at 25 volts dc. Available at a higher price with better performance and reliability.
	Small size low cost electrolytic used where high ripple current is not present. May be used for coupling or for decoupling but leakage current is high and reliability is low.
	Ceramic disc capacitor used where a low-cost, small, accurate capacitance is required with good temperature stability.
	Silvered mica capacitor used where a small, accurate capacitance is required with good temperature stability. More expensive than ceramic disc. The one on the left is coded Yellow spot, Purple spot, Red spot (4 - 7 - 2 giving 4700pF or 4n7F)
	Multilayer ceramic capacitor used where a high capacitance (up to 1uF) is required in a small space. Low voltage only and poor temperature stability but low impedance so very good for grounding low-voltage fast-risetime pulses.
	Tantalum bead capacitors used where a high capacitance (up to 100uF) is required in a small space. Low voltage only (63v max.) and polarised but low impedance so very good for grounding low-voltage fast-risetime pulses. Relatively expensive. The left capacitor has its value printed on but the right hand one is coded Red body with a Red stripe and a White spot. That's 22uF but I don't know why!
	Polyester film capacitors used where a low-cost medium capacitance (up to 4u7F) is required at a moderately high voltage with moderately good temperature stability and tolerance. Left hand example is resin-dipped (cheaper). Right hand example is resin-sealed into a moulded plastic box for better reliability. MKT capacitors are a good example of a type that use polyester called polyethylene terephthalate (PTPE).
	Polystyrene film capacitors used where a small capacitance is needed with low tolerance spread (2% or better) and good temperature stability. Drawbacks include relatively high inductance and cost and susceptibility to cleaning solvents. Yellow example is of the "extended" type which is better sealed against ingress of liquids. Polystyrene capacitors are most often used in tuned circuits (oscillators and filters) where frequency stability is important.
	Polypropylene dielectric capacitors offer very low dielectric losses and good temperature coefficient. Used in power electronics applications such as for mains interference suppression, switch mode power supplies and TV deflection circuits.
	Polycarbonate dielectric capacitors offer a low temperature coefficient and lower dielectric losses at high frequency. Most often chosen for temperature stability.
	Paper dielectric capacitors. Now rarely used because of unpredictable tolerance, susceptibtibilty to moisture and difficulty of manufacture compared with modern plastic dielectric types.
	More exotic types using, for example, PTFE (polytetrafluoroethylene) dielectric are used for demanding applications where the utmost temperature stability is required, however these offer no advantage in household or office appliances, being required only in aircraft/spacecraft or medical equipment where extremes of temperature are encountered.